A number of commercial applications require compact, efficient, and reliable laser systems in the ultraviolet wavelength range near and below 250 nm and also near and below 200 nm.
One of these applications is inspection for the semiconductor manufacturing industry. This includes inspection of wafers during processing, masks that are used for photolithography, and reticles. Currently, advanced semiconductor processing is using laser sources at 248 nm (KrF excimer lasers) and 193 nm (ArF excimer lasers) for photolithography. Laser sources near these wavelengths are needed for inspection applications.
Currently, there are no adequate laser sources for semiconductor inspection at these wavelengths. Excimer lasers that are used for photolithography have the proper wavelengths, but operate in pulsed mode with low repetition rates (less than about 1 kHz). These lasers are not suitable for scanning applications such as inspection. Frequency-doubled ion lasers (FreDs) can provide continuous wave (cw) outputs at various wavelengths near and below 250 nm (for example, 257 nm and 244 nm), but these systems are very large, inefficient, costly, and relatively unreliable. Diode-pumped frequency-quadrupled Nd:YAG lasers can provide cw output at 266 nm, which is rather far away from the desired wavelengths near 250 nm and 200 nm. Laser systems employing photons from 244-nm FreD lasers combined with photons from 1064-nm solid-state lasers provide output photons at 198 nm (1/244+1/1064=1/198), but these systems are very complex, inefficient, costly, and relatively unreliable. That approach also requires two separate lasers, which is undesirable for cost and complexity considerations. Further, for pulsed operation, it is costly and difficult to adequately synchronize pulses from different pulsed laser sources.
U.S. Pat. No. 5,638,388, issued Jun. 10, 1997 to Nighan et al., titled “Diode pumped, multi axial mode intracavity doubled laser,” is incorporated herein by reference. Nighan et al. describe an intracavity frequency doubling using Nd:YVO4 for lasing and temperature-controlled KTP for frequency doubling to obtain 532-nm wavelength light having a large number of axial modes such that the total continuous wave (cw) output power remains relatively constant.
U.S. Pat. No. 6,002,697, issued Dec. 14, 1999 to Govorkov et al., titled “Diode pumped laser with frequency conversion into UV and DUV range,” is incorporated herein by reference. Govorkov et al. used separate temperature-controlled enclosures for their non-linear crystals, and frequency quadrupled and quintupled 1064 nm wavelength light to obtain 266 nm wavelength light and 213 nm wavelength light, respectively, but these are longer wavelengths than desired for certain applications.
A paper by D. C. Gerstenberger et al., “Non-critically phase-matched second harmonic generation in cesium lithium borate,” Opt. Lett., 28, 1242 (Jul. 15, 2003), which is incorporated by reference, described efficient generation of 236-nm light (e.g., the fourth harmonic of a 946-nm Nd:YAG laser, in some embodiments) by use of noncritically phase-matched second-harmonic generation in cesium lithium borate. In some embodiments, noncritical phase matching provided approximately twenty times the nonlinear drive for second-harmonic generation than β-barium borate for 236-nm generation. In some embodiments, phase matching was accomplished at a crystal temperature of −15 degrees C.
Generation of ultraviolet light by use of diode-pumped 1-micron solid-state lasers and harmonic generation with nonlinear optical crystals has resulted in a variety of commercial laser products and industrial applications. The primary nonlinear optical crystals for generation of ultraviolet light are from the borate class. (See, e.g., T. Kojima, S. Konno, S. Fujikawa, K. Yasui, K. Yoshizawa, Y. Mori, T. Sasaki, M. Tanaka, and Y. Okada, Opt. Lett. 25, 58 (2000); D. J. W. Brown and M. J. Withford, Opt. Lett. 26, 1885 (2001); M. Oka, L. Liu, W. Weichmann, N. Eguchi, and S. Kubota, IEEE J. Sel. Top. Quantum Electron. 1, 859 (1995); and J. Knittel and A. H. Kung, IEEE J. Quantum Electron. 33, 2021 (1997), each incorporated herein by reference.) Borate-class nonlinear optical crystals include lithium triborate, β-barium borate (BBO), and cesium lithium borate (CLBO). Recently, average powers near 20 W at 266 nm (Kojima et al., supra) and near 15 W at 255 nm (Brown et al, supra) were obtained with CLBO. Conversion efficiencies for these systems approached 30%.
In general, the nonlinear drive for the borate crystals for fourth- and fifth-harmonic generation by use of 1-micron neodymium lasers is relatively low. This is so primarily because of the relatively small value of the nonlinear coefficient compared with those of crystals (such as KTP, lithium niobate, and periodically poled materials) for generation of visible light, and also because of Poynting vector walk-off. Thus, efficient nonlinear conversion that uses borates requires high-peak-power lasers or power enhancement in external resonators (for example, Oka et al., supra reported 52% efficient continuous-wave generation of 266-nm UV light when they used BBO in an external resonator, and Knittel and Kung, supra, used an external resonator and a diode-pumped Q-switched Nd:YAG laser system to generate 266-nm light with 30% efficiency). Chang et al. (L. B. Chang, S. C. Wang, and A. H. Kung, Opt. Commun. 209, 397 (2002)) utilized a similar scheme to generate the 213-nm fifth harmonic at 7.5% efficiency.
What is needed is an efficient fourth and/or fifth harmonic frequency up-conversion from approximately 1000 nm pump light.